Refining of volatilizable metals



Dec.,2l, 1948. H. F. FISHER REFINING OF VOLATILIZABLE META-LS Filed Nov.5, 1946 2 Sheets-Sheet 1 INVENTOR. HAPMON F. FISHER Dec. 21, 1948.

Filed Nov. 5} 1946 H. F. FISHER REFINING OF VOLATIL IZABLE METALS 2Sheets-Sheet 2 FISHER AGENT Patented Dec. 21, 1948 1 1 :Harmon FHFiSher,Sunnyvale, Calif.,- assignor. to i The Permanente Metals Qorporation,Oakland,

Califl, a corporation of Delaware.

' -This invention relates to a method for recovering volatilizablemetals from materials con 'taining "the sam'e.= More particularly, itrelates.

.toii thei vaporization of volatilizable metals from substantialamountsof non-volatilizable impurities by a method wherein the heatrequire'd'to vapot iae th'e -metal aisi supplied directly to thematerialicontaining the metal by means of a carrirgas i A r Theinvention is applicable generally tothe refining of volatilizablermetalssuch as--zinc, the 'alkali metals, the alkaline earth metals,"etc.,which" may be separated by vaporization; from concomitantnon-volatilizable impurities. For purposes of illustrationfhowever. the.invention will be "described with particular reference to thetreatmentbf magnesium; It is especially suited to the recovery ofmagnesium from carbothermic -magnesium=dust. M 1 v In the carbothermicprocess for the production of meta'llic magnesium a 'mixture ofmagnesium ox'ideand carbon'is subjected to a' temperature i-n the orderof: 2000 0. whereby magnesium and :carbon monoxide are formed accordingto the following reaction: i i

" MsQ+QA=M ---'-:Si-nce this reactionzis reyersible it is necessary tocool the magnesium and carbon monoxide to :a point'where they arestable; in the presence of feachz other. This cooling or chilling toperation :is rapidly effected withthe aid of large quantitiesioiaauchilling medium such as naturalgas, liquid hydrocarbons, hydrogen.or certain of the noble gases: -aHowever, even when good chilling. ef-.ficiency, is achieved there is some reversion of mthesreactionsothatthe product recovered com- -;.prises a mass ofabout fifty per centmetallic ;magnes ium with the remainder magnesium oxide ;and;carbon inthe ratio of about 60 to 40. 2 There;-

are also relatively small amounts of carbides present inaddition tolesser amountsofcalcium,

si-liqmri; .sodium, .ir0n.;and aluminum anditheir ,eoxidesy.Theqmaterial is finely divided and the individual particles rangeinsize forthe most i part from several microns to a small fraction ofone micron. In theindustry this product is usually referred to as.carbothermic magnesium di t; 13

The ecovery of and team magnesia of Application November 5, 1946, SerialNo. 707,819

, 9 Claims. (c1. s- -cs) use of special: equipment, and asa consequenceit. must be handled with considerable care and precaution In the processof purifying metallic mag.- nesium; of; the type produced by thecarbothermic process, thecoperation in the past has beentoplacethelfinely. divided magnesium together with its accompanyingimpurities .(i. 'e. magnesium oxide, magnesium carbide, carbon, silica,etc.) into apelleting machine and then to feed the resulting pelletsinto aretort; The retort was hermetically sealedand then loweredintoapitfurnace and evacuated to decrease the. distillation"temperature. Any volatile binders and impuritieswere'first distilled ofiat. a low temperature and then the temperature was raised until themagnesium metal was volatilized. Because of the. evacuated interior ofthe. retort and. be-

cause of the, low heat conductivity of the charge stockv and the high.heat capacity of the-retort,

the, process, was extremely slow. The; retorts were subjected to hightemperatures because of the necessity of transferring heat through theshell of the retort. and thence by radiation through the'i poorlyconducting pieces of charge stock containing the magnesium-.1. The.capital investment win heavyzplant equipment was extremely high-andequipment life: was short because of thejsevere conditionsimposeduponit.

It is an object of the presentinvention to provide a method for thedistillationof volatilizable metals'whereby the heat required for thedistillation of the metal-from the charge stock is carried directly tothe charge stock by a carrier gas. An additional object is to provide "amethod for the distillation of metals which is rapid and .etficient,-and:which does not require. the use: of expensive plant equipment or severeoperating conditions that are highly destructive to plant equipment.

,In practicing the present inveritionga stream of carrier gas; isfirstheated to a temperature above the'boiling temperature of thevolatilizable gmetal' sought in pure .form, and then the heated carriergas is circulated through a bed of-the chargestockcontained in aninsulated chamber or retort. Continued circulation of theliotcarrier'gascauses a rapid rise in the temperature of the charge stockuntil the boiling temperature of the volatilizable metal is approached.At these elevated temperatures the metal'exhibits an appreciable vaporpressure and, 'inthe presence of the stream of carrier gas itvaporizesf-at ;a rapid rate. The metal vapors are conveyed'by thecarrier gasto a condenser where the metal is 3 condensed to the liquidor solid state .and separated from the carrier gas. The recoveredcarrier gas is recycled under pressure through a suitable heating zoneand fed back through the charge stock.

In the event that any dust is mixed with the charge stock pellets,fragments, or other agglomerates it fnia'ly be blown out o'fj'thchafg'estock" early in*the"lieating cycle by seiieral-blast of car= rier gas.The dust particles picked up by the suitable dust separating means.

In a modified embodiment oi the preseiitinverr tion, a preheating stepmaybe-interposedwer (or from other recovery sources), is circulatedthrough the charge stock agglomeratestolieat- 2O obviated, and themetazl walls and heat insulatthem to a temperature somewhat below themelting: point of the. metal .tohbe'tvaporizedz As describedabovervolatile binderssuch mineral permits a simple and economic"separation of dust rromtheicarrier gas when such separation' of=-dustis necessary. If appreciable amoun'ts of dust are present with theimetalvapors fin the carrier gas that enters the condensen'the condensermaybe- :come' r fouled T: by accumulated dust' particles; and the rateof heat transfer to the cooledcondensing surfaces will be decreased.v Inaddition,-the en trained. dust may contaminate the 1 recovered um metalwith impurities ancl'thereby decrease-its Ivalue.

Dust's'eparators,',such asfiltrs and cyclones,

employed in" the process-of the inventionmust be amaintained attemperatures abovei'the condenser temperatures. By the use :of a. dustcollector in the primary preheat? ga'ssstream, lower temperatures may beused on thedust collector. The pri- 'mary preheat :gas stream, which isbetter adapted for freeing-the charge "stock of dust, may be sentthroughthe'.pelle'tedcharge stock at a higher velocity than thesecondary vaporization carrier gas stream, which carries;metalfvapors Ithat mustibe condensed.

The carrier-gases employed in'th'e present pro'c-= -ess are prefera'b'lyinert. Hydrogen, becauseof its-inert character to' magnesium and. mostofthe volatilizable metals, has been found to be'very satisfactory. Suchinert-gases as helium, and argon may also be used.

Pressures above atmospheric are' conveniently used in the process.thereby minimizing: the danger of'air infiltration into the carrier'gas-"system. The process may, however, be operated at pressures near o'rfar'belowatmospheric pressure" byrusing suitable precautions to preventleakage 'ofair-intotl'ie system, i z

v The carrier gas ma befl'ie'ated by 'uny Suitable means such asrefractory pellets'ofth'e eontained in Royster 'st0ves,'o'r bylectricalesistance elements, similar-to enhances-ma eof silicon carbide: whichmay be saf ly-operated "up t'o'ZSUO" F; Metallic resistors may be' used,prov ded the upper temperature lin'iits' -aretvithifi the safe workingrange of the anoy.--

heating may rio also be accomplished by radiant heater tubes made ofhigh temperature all-y. These tubes are sealed off from the carrier gasin the heating furnace. Combustion of fuel within the heater tubessupplies the necessary heat.

Since all the high temperature heat required in the instant process issupplied directly to the carrier! gas from within' the" encwsed: systemrather than frdm heat' supplied' tbthe' outside walls of portions of theequipment, none of the carrier gas blasts are then separated fromthecarrier gas by the use of cyclonefilters or otherri-ietal walls ofthe equipment-such as retorts, re-

,tort shells eteare at high temperatures during the operation oftheprocess. It is, therefore, possible to line the :hot gas ducts and thevaporiza- 5"tion retor'tswitlihigh temperature refractory and'i'irsulatiirgliningssuch as, for example, carbon i r 1-rthe hottestzones, and magnesia in the low temperature zones. Thus heat losses arereduced, triene'ea for expensive heavy plant equipment is ing material:are' not destroyeclabyroxldatieii or reduction reactionsr. Thecarrierwg'asprocesszof theinventiohfmazytbe operated irr ei-ther:batchor continuoussmanner.

. Examples fofrboth modes at operationl'wlll-ibesdexscribed:

Fig: 1 is :a descriptive :drawing: of a -dis'tillatlon'or't'vaporizatiom retort .which may be used ;-in. c0n-, nection with abatch type operation ofstheinven- 'tio'n.-

A gas-sealed chargingrhopper' l0; into-which funnel 'l l empties;isequipped with :plugzvalves'tl'z and I3 and-ismountedontopofr'retortt-lQ5 Between-the .top of retort l 4 and plugvalvel'l 3 there ,3; is installed a' spe'ctacle'slide valve:l5.

Retort I 4=iscomposed of atop.=-section? I Grantee bottom section 1| 1:Both top; ands=bottom sectlons of the: retort are composed of join-ed.-eylind-i ieal sections and truncated cone sections asz'shown in sFig:1;- Ifhe-wallsofsection Morton-section l6 areslightly inclinedinwardfromthe verticalland form' atruncated l conerather: than a cylinder.Gpening into thetop. andebottom'isections: oflth'e retort are inlet:ducts l9 andlllr respectively. Spectacle slide valves 2|, 22andi2'3:arafitted to ducts l9 and zlllandto the outlet at the bottom ofthe retort. Highterfipfatiife'alloy plug cocks 24; 2.5;: and" 2 6areinstalle fzadjaeentztoitnazsfieota cl'e slidecvalv'es.

5o 7 The: wallsnofthetop: ancl'bottomasectronss [5 and" I6" of theretorti and: of ducts are: composed of i' a. steel 1 'outen'stillilin'ed withirefr'actory. -'carboni=.brick liniiigri'baclied up: withcarbon: black heat-insulation:

wand; 20

Near the tbp of the bottom-'rsectiomtFof theretort tare fastened: two-hinged isemi eirc'ular irate bars 21.: The:grateebarssiare niadeoffing-1i exiliieratureialloyv'andi' the: forged hinges on thegratestare oil -cooledr The spctaeleeslile valves 6 be usedtovadvantage:

I Charging dry pellets-fragments; other alijiiierated forms ofcarbothermic magnesium-(Hist 0 fareenareedanteruiirr rf n until"'it"'is"filledf 'rne ewe-rooted;into-charging opiiei l'o valve 12."Irrtliejhopiir tlie' cli'afge 5 is urge-a; free-trait with the earner 56wnememgrngasedfiifiiete; tia-itelz ins -teen passing a ae-upward throughth 7 Preheatz'ng If preheating is practiced, a preheating carrier .gasstreamis passed through the bed of charge stock. Valves l3, I5, 23 and26 are closed and valves 2|, 22, and are opened to pass thehot gasthrough duct 29 uputhrough the cold charge stock and out through ductl9. By this. process the retort and charge are warmed up to a uniformtemperature of about 1204? F., the melting point of magnesium. At thistemperature the vapor pressure of magnesium is less than about 2 mm. ofmercury and only a small amount of vaporization occurs. i

During this preheating step loose dust and broken pellets are swept out.of the pellet 'bed and into a dust separator. It is preferable, at

.the beginning of the preheating stage, to briefly scavenge the retortcharge with a high velocity blast of carrier gas for the purpose ofremoving dust from the charge bed. If the chargestock contains avolatile material such as a hydrocarbon binder this material isconveniently vapor- ,ized from the charge during the preheating step.

Vaporizing After the preheating cycle has been completed, the preheatinggas stream is shut ofi and a stream of vaporizing carrier gas, having ahigher temperature, is circulated through the retort in the same manneras described for the preheating carrier gas stream. This circulation iscontinued untilsufiicient heat is supplied to vaporize the metal in thedry pellets, fragments or other ag-,.

glomerated forms. Practically the entire metal contents of the chargestock is vaporized and carried away in the hot carrier gas to acondenser (not shown) where the magnesium is recovered by condensing itto the liquid or solid state.

The temperature of the carrier gas used on magnesium charge stock shouldnot be appreciably below 2015 F. for rapid vaporization of the metal.When the temperature of the carrier gas is appreciably below 2015 F. amuch larger vol- 'ume ofcarrier gas-is necessary'to vaporize a givenmass of magnesium then when carrier gas having a temperature above 2015F. is employed.'

Carrier gas temperatures as high as 2035 to 2050 F. are preferable forrapid rates, of vaporization and have the additional advantage thatsmaller rate of gas circulation will, in turn, depend upon the depth andtemperature of the charge stock as well as the size of charge stockagglomerates, radiation conditions, etc. The optimum rate of carrier gascirculation is determined experimentally for each set of conditions.

Residue cooling After the vaporization cycle is completed, the

carrier gas stream is shut off and a stream of cold carrier gas iscirculated through the hot bed of residue in the retort in the samemanner as described for the. previous gas circulation cycles. The coldcarrier gas cools the hot residue and is itself heated sufficiently tobeused as a preheating carrier gas stream in another retort charged withcold ag lomerated charge stock. The cold carrier gas stream iscirculated until the residue bed is cooled to 1200 F. or any otherdesired temperature.

Discharging After the residue and the retort have been cooled to thedesired temperature the cooling gas stream is shut ofi, valves 2l, 24,22 and 25 are closed and valves 23 and 26 are opened to a residuereceiver (not shown) which was previously purged with a blanketing gas.grate bars .21 are then lowered as shown by dotted lines in Fig. 1 andthe residue drops down out of the retort into the residue receiver. Theretort is then ready to be charged again as described above. I

The above cycles of operations are essentially the same for the recoveryof other volatilizable metals than magnesium except that difierenttemperatures are employed. 7

Fig. 2 is an illustrative drawing of an apparatus which may be employedto continuously evaporate volatilizable metals in accordance with themethod of the invention.

In operation with agglomerated carbothermic magnesium dust in the formof pellets, for example, the gas tight, inert gas filled surge chamberII] is continuously charged with stock from .gas locked feed bins (notshown) through pipe H. A continuous, gradual movement of the chargestock is maintained downward in rectangular retort tube l2, which isabout six times as wide as it is thick. As the charge stock moves intothe zone of preheat carrier gas duct 13, it is preheated by across-stream of inert preheat carrier gas (having a temperature of about1250" F.) passing through duct l3 and into cyclone separator It. Incyclone separator I4, any. dust that is picked up from the downwardmovingcharge by the preheat carrier gas is separated from the gas, andrecycled through a pelleting or agglomerating station to the feed bins.The preheat carrie r gas leaving cyclone separator; l4,

through pipe [5, may be recirculated as cooling gas in the mannerdescribed below. 1

In its continuous movement through retort tube [2, the preheated chargestock next enters the zone of vaporization carrier gas duct 16, where itis contacted by a cross-current ofhot inert vaporization carrier gashaving a temperature of 2100 to 2150 F. or more. While passing slowlythrough the vaporization carrier gas cross-current at the mouth of duct16, the charge stock is heated sufiiciently to cause substantiallycomplete vaporization of the magnesium from the stock into the stream ofhot vaporization carrier gas. The magnesium is recovered from the hotvaporization carrier gas stream by passing the stream through acondenser (not shown) wherein the magnesium vapor is condensed to eitherthe liquid or solid state.

From the mouth of duct I6, the hot residue fragments or agglomeratescontinuously pass to the zone at the mouth of duct 11. Here they give upa substantial portion of their heat content to a cross-current ofcooling carrier gas. This cooling carrier gas may be circulated fromHingedmerges-2s .yeionex separator outlet! pipe: i51 After" it. isheatedzibyither'esiduenit'imay be -pa'ssedtinto pre heatroarrierrgasduet; l 3

The cooled". residue? agglo'merates: are: moved downwardn continuously"through? am adjustable discharge mechanism: (not shown) and i into aresidue: receiver (not: shown): connected: tow-the bottom of retort-tube I21:

Iiiisimilar batch'ior continuous. methods of 10pera-tion, othertolatilizabl -nietalsisuehias sodium, potassium, rubidium, .cesi-um,barium; strontium, calcium and Zinc,.may. be recovered from carbothermicreduction dusts, and the like, and the revenue-m is' riotiiitende'd' to'be limited 1 to the i eiz it is particularly advantageous mreceverifigthelatterriietal shite-several of th'e diffieulties r'ieour-i-t'eredwith magnesium Manet-encountered with an other readil-ytolatiliiablemetals".

The processor the invetition' is not intended 'l-imitdto pelletedcarbothermic dust of iiable metals, Bilit' is" applicable t theglomerates or fragments composed of volatihzabl'e metals such as Zinc,mercury, barium, strontium, sacrum; sodium, potassium, rubidium, cesium,magnesium,- cadmium, bismuth, arsenic, antimony, radium, se1eniu'm,etc., and containing nonwolatiliz'able impurities such as carbon,mtaloxide's, and other inorganic compounds.

1. In the method of refining a mixtu're of a -vol-atilizab'le metal andless volatile solid impurity, wherein themixture is heated toatemperature suflicierit to vaporize themetalbut insufficient tovaporii'e the impurity, and the metal vapor is separated fromthe-unvaporized impurity; the impr'oveinent which comprises: supplyingthe heat er vaporizationof the metal directly-t0 a bed ofagglorheratesof 'said mixture by meansof a preheatdiriert gas, bypassing a stream-ofthe hot preheated gas through said bed or the mixtureat a temperature and at a ratesu'c'h that the metal "exerts asubstantial vapor pressure,- thereby va eri'zm the metal from saidmixture into- 1 said stream ofhot'ihert'gas at a substantial rat-e'bii'virtue of'the heat transferred'from' the hot preheated gas to thebed of the mixture; and separating the metal from the hot inert gasstream.

2. The improvementof-claim 1, wherein said ls's' -volati1e impurity iscomposed largely ofthe oxide of the volatilizablemetal and a reducingagentcapab'le of'reduc'ing the o'xide'to the metal at a hightemperature, and the temperature of" themixture of iiiertgas'andfihelydivided Solids is' we1l below temperatures at which substantial reduetidm of; the oxide by." thereducing; agent occurs butiissufficientlyhigh to producesubstam tial :vapori'zation of thefmetal; i

3-: In athe'meth'od of refinmgaimixture omnag nesium metal and 7 less:volatile sol-id" impurity composed largelmofmagnesium oxide: and carbon,wherein the mixture is heated to a temperature sufficient to vaporizethe metal but insufiicient to 'vaperize-thedmpurity, anelithemetaivapor: is

separated fro'rnitlie unvapori'ze'd' impurity; the 1m:- provemeht whichcor'riprisesf supplyin'g 'the heat e'ri vaporization of the? metalidire'etly to' a bed of a'g'gl'emerates 'of said mixture, bypassinga'stream ofin'ert' -ga's; preheated to a temperature 1 within the ranaeof aboutc2015 -"to 2050 F;, through said bed of the :mixture at a rate"such thitithe metal exerts asubstantialwapor pressure;therebyzvaporizingrthametalfrom said mixture: into said stre'an 'i'ofhot -inert=;eas at a substantial rate by virtue of the heattransferredfrom the hot-Cpre heate'dgas twthe'ibed of the mixture';andis'eparating the metalifrom the'hotainertigasstream:

4 The zimprovemerit of f claim 3-; wherein said inerizgas is -hydrug en;

5: The improvement of 1 claiml3'; wherein irrertigas is alnoble' 'gas'.

6." Thea m'etho'dwf refining-1a solid mixturenof volatilizable metal"and. less'volatile impurity, which comprises e'stahlishing'a'cyelic fiow'of a' gas Said .whichiisinert with respect tot't-he' metal, .heatingthe gas during the course ofits flow to a temperature at which the metalexerts a substantial vapor pressure but below theztemperatures -ofsubstantial vaporization of said impurity, passing a stream of the' hot'inert gas, after itahas been thus preheated; through a-bed ofagglomerate's of said mixture at such a rate to effectsubstantial heatUNITED sTM-F's PATEN' 7 Namev Date Ogorzaly l Jan. .29, :1946

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